This invention relates to antennas, and more particularly to reducing the effects of shadowing from a multipath environment, using space diversity and coding.
Data sent from terminal to base, or vice versa, are often shadowed. Shadowing is a function of time, and may be caused by buildings, foliage, vehicles, people, motion of the terminal, etc. Shadowing is the blocking, or attenuating, of the transmitted signal. Shadowing may occur in fixed or mobile systems, and can vary slowly or quickly depending on the situation.
While shadowing has an effect which is similar to multipath, the causes and statistics of shadowing may be very different. For example, the presence of a building may result in total shadowing, independent of time, while multipath, caused by numerous multipath returns, produces a Rayleigh or Ricean fading distribution. Fading due to shadowing and multipath may be reduced by adding a receiver antenna to increase receiver diversity.
Coding techniques using space diversity as well as time, are known as xe2x80x9cspace-timexe2x80x9d codes. In the prior art, with a multiple antenna system, the input to each receive antenna is assumed to have Rayleigh fading. A problem with multiple antenna systems is that a particular antenna output may be shadowed by 6 dB or more to a particular receive antenna. Such shadowing leaves the other antennas to receive a desired signal, effectively destroying one source of data.
A general object of the invention is to reduce the effects of shadowing and multipath in a fading environment.
Another object of the invention is to improve performance of a spread-spectrum communications system.
An additional object of the invention is to increase capacity of a spread-spectrum communications system.
A further object of the invention is to minimize fading and enhance overall performance in a spread-spectrum communications system.
According to the present invention, as embodied and broadly described herein, an antenna system is provided employing space diversity and coding, for transmitting data having symbols, over a communications channel. The transmitted signal passes through a communications channel having fading caused by multipath as well as shadowing.
In a first embodiment of the invention, the antenna system comprises a forward error correction (FEC) encoder, an interleaver, a demultiplexer, a plurality of spread-spectrum devices, a plurality of transmit antennas, and a plurality of receiver subsystems. Each receiver subsystem includes a receiver antenna and a plurality of matched filters. The receiver system further includes a RAKE and space-diversity combiner, a multiplexer, a de-interleaver, and a decoder.
The FEC encoder encodes the data using an error correction code to generate FEC data. The interleaver interleaves the symbols of the FEC data to generate interleaved data. The demultiplexer demultiplexes the interleaved data into a plurality of subchannels of data. The plurality of spread-spectrum devices, spread-spectrum processes the plurality of subchannels of data with a plurality of chip-sequence signals, respectively. Each chip-sequence signal of the plurality of chip-sequence signals is different from other chip-sequence signals in the plurality of chip-sequence signals. The plurality of spread-spectrum devices thereby generates a plurality of spread-spectrum subchannel signals, respectively. The plurality of transmit antennas radiate, at a carrier frequency using radio waves, the plurality of spread-spectrum-subchannel signals over a communications channel as a plurality of spread-spectrum signals. The plurality of spread-spectrum signals could use binary phase-shift-keying (BPSK) modulation, quadrature phase-shift-keying (QPSK) modulation, differential encoding, etc., and other modulations, which are all well known carrier modulation techniques.
The communications channel imparts fading on the plurality of spread-spectrum signals. The multipath generates a multiplicity of fading spread-spectrum signals. The fading also may include shadowing.
The plurality of receiver subsystems receive the plurality of spread-spectrum signals and the multiplicity of fading spread-spectrum signals from the communications channel. Each receiver subsystem has the receiver antenna for receiving the plurality of spread-spectrum signals, and the plurality of matched filters. Each receiver antenna in the plurality of receiver antennas is spaced from other receiver antennas in the plurality of receiver antennas preferably by at least one-quarter (xc2xc) wavelength, and preferably as far apart as practicable. The present invention includes spacings less than one-quarter wavelength, but with degradation in performance The plurality of matched filters has a plurality of impulse responses matched to the plurality of chip-sequence signals, respectively. The plurality of matched filters detect the plurality of spread-spectrum signals and the multiplicity of fading spread-spectrum signals, as a plurality of detected spread-spectrum signals and a multiplicity of detected-fading spread-spectrum signals, respectively.
A plurality of RAKE and space-diversity combiners combine the plurality of detected spread-spectrum signals and the multiplicity of the detected-fading spread-spectrum signals from each of the plurality of receiver subsystems, to generate a plurality of combined signals. A multiplexer multiplexes a plurality of combined signals thereby generating the multiplexed signal. The de-interleaver de-interleaves the multiplexed signal from the multiplexer, and thereby generates de-interleaved data. The decoder decodes the de-interleaved data.
As an alternative, a preferred embodiment is to select the received version of each received chip-sequence signal at each antenna and combine them in a RAKE. In this embodiment, the space and time combining of each channel from a respective chip-sequence signal occur in a single RAKE receiver. The total number of RAKE receivers is equal to the number of chip-sequence signals, or one or more RAKEs could be time multiplexed to represent the number of chip-sequence signals.
A second embodiment of the invention has an antenna system for transmitting data having symbols over the communications channel having fading caused by multipath and shadowing. In the second embodiment of the invention, as previously described for the first embodiment of the invention, a multiplicity of delay devices is coupled between the interleaver and the plurality of spread-spectrum devices, respectively. A first signal of the plurality of signals of the interleaved data need not be delayed. The other signals of the plurality of signals of interleaved data are delayed, at least one symbol, one from the other, by the multiplicity of delay devices. Each delay device of the multiplicity of delay devices has a delay different from other delay devices of the multiplicity of delay devices relative to the first signal. The multiplicity of delay devices thereby generate a plurality of time-channel signals.
The plurality of spread-spectrum devices has a first spread-spectrum device coupled to the interleaver, and with the other spread-spectrum devices coupled to the multiplicity of delay devices, respectively. The plurality of spread-spectrum devices spread-spectrum process, with a plurality of chip-sequence signals, the first signal and the plurality of time-channel signals as a plurality of spread-spectrum signals. The plurality of transmit antennas radiate at the carrier frequency, using radio waves, the plurality of spread-spectrum signals over the communications channel.
The communications channel imparts fading due to multipath and shadowing on the plurality of spread-spectrum signals. The multipath generates a multiplicity of fading spread-spectrum signals.
The plurality of receiver subsystems receive the plurality of spread-spectrum signals and the multiplicity of fading spread-spectrum signals from the communications channel. Each receiver subsystem includes a receiver antenna for receiving the plurality of spread-spectrum signals and a plurality of matched filters; the plurality of matched filters has a plurality of impulse responses matched to the plurality of chip-sequence signals, respectively. The plurality of matched filters detects the plurality of spread-spectrum signals and the multiplicity of fading spread-spectrum signals, as a plurality of detected spread-spectrum signals and a multiplicity of detected-fading spread-spectrum signals.
A RAKE and space-diversity combiner combines the detected spread-spectrum signal and the multiplicity of detected-fading spread-spectrum signals from each of the plurality of receiver subsystems. This generates a plurality of combined signals. The FEC decoder decodes the de-interleaved signal as decoded data.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.